1. Field of the Invention
This invention relates to exterior curtain wall mullion anchoring system design.
2. Description of the Background
An exterior curtain wall system consists of three major components, namely, wall panels providing weather protection, mullions providing structural support to the wall panels, and mullion anchoring systems providing a structural connection between the mullions and a building structural element. Mullion anchoring systems carry the dead load weight of the wall panels and transfer the load to the building structure, typically at the building base or at intermediate floor slabs. Mullion anchoring systems also absorb positive and negative wind loads acting on the wall panels.
Mullion anchoring systems also must allow for construction tolerance adjustments in all three directions (i.e., up/down, left/right, and in/out). The acceptable construction tolerance for curtain wall, typically ±⅛″ (3.2 mm) in all directions, is much tighter than the acceptable construction tolerance for the building structural elements, typically ±¾″ (19.1 mm) in the up/down direction, ±1″ (25.4 mm) in the left/right direction, and ±1″ (25.4 mm) to ±2″ (50.8 mm) in the in/out direction. Mullion anchoring systems must be designed to absorb these construction tolerances. The three way construction tolerance adjustments are executed in the field individually for each mullion anchoring location.
Mullion anchoring systems may be categorized based on where they are secured to the building structure. For example, mullion anchoring systems may be secured on the face of a floor slab (i.e., edge of slab or slab edge application), on top of a floor slab (i.e., on-slab or top of slab application), or to a support beam or column.
Mullion anchoring systems secured to a concrete floor slab may be further categorized based on how they are secured to the floor slab. For example, a mullion anchoring system may be secured to a concrete slab using concrete anchor bolts installed after the concrete is cured, secured by welding to a weld plate embedded in the concrete when the concrete is poured, or secured using special T-bolts secured to a slotted anchor channel (also referred to as “cast-in channels”) embedded in the concrete when the concrete is poured. Mullion anchoring system components embedded in the concrete floor slab when the concrete is poured are commonly referred to as “embeds.”
A slab edge embed is commonly used to anchor mullions in a stick system curtain wall. When a typical slab edge embed is used, the mullion anchoring system includes the slab edge embed and mullion connection clips (also referred to as brackets) connecting the embed to the mullion. The clips typically are a pair of L-shaped angles, one on each side of the mullion, each with an anchoring flange secured to the embed and a protruding flange secured to a side of the mullion. Three-way construction tolerance adjustments are normally provided by vertical slotted holes in the mullion for up/down adjustments, horizontal slotted holes in the protruding flange of each mullion connection clip for in/out adjustments, and horizontal slotted holes in the anchoring flange of each mullion connection clip for left/right adjustments. The slab edge embed may have two threaded steel rods (acting as anchor bolts) protruding horizontally outside the floor slab edge for structural bolted connection to the anchoring flanges of the mullion connection clips.
Alternatively, a slab edge embed with an anchor channel (sometimes called a cast-in channel) may be used. If a cast-in channel is used, the mullion connection clips are secured to the channel using a field-installed anchor T-bolt. Left/right adjustments can be made by positioning the anchor T-bolt at the desired left/right location within the channel. Up/down adjustments can be made by using vertical slotted holes either in the mullion or in the anchoring flange of each mullion connection clip. In/out adjustments can be made using a horizontal slotted hole in the protruding flange of each mullion connection clip.
In a mullion anchoring system with a slab edge embed, the up/down adjustment must be done with a temporary dead weight support first, followed by simultaneous adjustments in the other two directions before tightening up all connection bolts. For erection safety and quality, the above procedures require handling relatively light weight mullions without attached wall panels, such as in a curtain wall stick system or airloop system.
Some functional disadvantages of slab edge embed anchoring systems include: (1) They require punching or notching through the slab edge concrete stop before pouring concrete for the protruding threaded steel rods for the connection bolts or for the exposure of the anchor channel; (2) It is extremely difficult to remedy incorrectly located embeds after the concrete slab cures; (3) In case of incorrectly located holes in the mullion, the mullion must be re-fabricated in the shop, causing potential job delays; (4) Quality control inspection is more time consuming since the anchoring components are outside the slab edge.
Some functional advantages of a slab edge embed anchoring system include: (1) The embed condition likely will not be damaged or displaced by the concreting operation; (2) Only light hoisting equipment is required to erect the mullions.
Some structural problems of a slab edge embed anchoring system include: (1) The anchor bolts are subjected to both shear and tensile stresses due to dead and cyclic wind loads, causing potential stress fatigue; (2) Use of slotted holes for construction tolerance adjustments means the structural connection strength against wind load reaction becomes a function of the distance from the connection bolt to the center of the slotted hole; therefore, either the worst condition or a higher safety factor must be considered; (3) Using slotted holes for left/right adjustment results in uneven wind load reactions on the double L-shaped mullion connection clips causing twisting of the mullion, producing potential sealant line failure or wall panel connection failure.
Mullion anchoring systems that include an on-slab embed are commonly used for a unitized system where heavy curtain wall units are involved. In a typical on-slab embed anchoring system, an anchor channel is partially embedded in a concrete floor slab when the concrete is poured. A bracket is secured to the anchor channel using anchor T-bolts, and the bracket is engaged with mullion connection clips that are fastened to the mullion.
Three-way construction tolerance adjustments for this type of on-slab embed are normally executed by the following procedures: (1) Hoist the curtain wall unit to be erected and engage it to the adjacent erected unit to form the vertical wall joint; (2) Position the bracket at the desired right/left location along the anchor channel; (3) Using slotted holes in the bracket, move the bracket to the desired in/out position for engaging it with the mullion connection clips that are attached to the mullion; (4) Lower the wall unit down to cause simultaneous structural engagements between the mullion connection clip and the bracket, and between the wall unit and the erected unit below to form the horizontal wall joint; (5) Fix the bracket in position by securing the anchor T-bolts to the anchor channel; (4) Drop down the unit to completely engage the horizontal wall joint below with the weight being supported on the bracket; (5) Use a vertical set-screw in the mullion connection clip to accomplish the up/down horizontal wall joint line to be within the acceptable tolerance range of ±⅛″ (3.2 mm); (6) After final vertical joint gap adjustment if necessary, secure the unit against horizontal walking and release the hoist.
Some functional disadvantages of an on-slab embed anchoring system include: (1) It requires heavy hoisting equipment for the erection; (2) It is difficult to maintain the design position of the embed due to the fact that the embeds are often inadvertently kicked out of position or buried inside the slab during concreting operations, and it is costly to remedy the problem of incorrectly located embeds.
Some functional advantages of an on-slab embed anchoring system compared to a slab edge embed anchoring system include: (1) Various remedy options can be used for incorrectly located embeds after concrete curing; (2) It is easy to execute reliable field quality inspection due to the on-slab location of the anchoring system.
Some structural problems of prior art on-slab embed anchoring systems include: (1) The dead load reaction is transmitted from the mullion connection clip to a point on the bracket that overhangs the floor slab edge, and the overhanging distance depends on the amount of in/out construction tolerance adjustment. This creates a variable bending moment on the bracket at the slab edge and a variable uplifting long term load on the anchor T-bolts that secure the bracket to the anchor channel embed. Due to the variable bending moment and uplifting long term load, the bracket and the anchor T-bolts must be designed for the condition of maximum outward construction tolerance adjustment. (2) The up/down tolerance adjustment is normally provided by a set-screw type of device at the dead load supporting point in the mullion connection clip. The connection strength between the mullion connection clip and the bracket varies due to the change of the depth of structural engagement between mullion connection clip and bracket caused by the up/down tolerance adjustment. (3) The combined dead load and wind load reactions produce both a pull-out force and a shear force on the anchor T-bolts. To obtain adequate structural strength of the anchor channel embed, a minimum distance from the embed to the slab edge and a minimum embed depth are required. (4) The maximum up/down tolerance adjustment that can be provided by a set-screw type of device in the mullion connection clip is rather limited, typically ±¾″ (19.1 mm), while the practical up/down construction tolerance of the slab edge surface is often in the range of +1.5″ (38.1 mm). It is cost prohibitive to solve this problem by relocating the mullion connection clip in the field since it will significantly slow down field productivity. Therefore, it is common field practice to level from the high points on the slab surface, typically at the column locations and to use shims on the bracket at the low points to fulfill the maximum ±¾″ (19.1 mm) up/down adjustability. The impairment of anchoring strength due to shimming is largely ignored.
In prior art on-slab mullion anchoring systems, the uplifting force on the anchoring device generated by dead load is a long term load. To resist this long term uplifting force, prior art systems use anchoring devices secured to the concrete floor slab either using large anchoring bolts or components embedded in the concrete when the concrete is poured.